I recently learned that osmosis occurs when there is a semipermeable membrane between pure water and a solution of water. The pure water then fills up the solution's side until, as my textbook put it,
"The pressure exerted by this column of solution counterbalances the pressure exerted by the water moving through the membrane from the pure water side."
My question is how does water just naturally want to move from one gradient to the other? Doesn't water move through the membrane both ways? Also, why does water "want" to go to the more concentrated side when the particles move at random?
In other words, why doesn't the flow of water molecules depend on the number of water molecules in the membrane instead of the concentration gradient?